1. Field of the Invention
The present invention relates to an optical transmitter and an optical transmission system that can minimize the deterioration in the transmission quality due to the chromatic dispersion of an optical transmission medium such as an optical fiber, or due to the interaction between the chromatic dispersion and nonlinear optical effects.
This application is based on patent application No. 2001-199467 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
An RZ (return-to-zero) optical intensity modulation format used along with phase modulation has been proposed having the object of minimizing the deterioration in the transmission quality due to the chromatic dispersion of an optical transmission medium such as an optical fiber, or due to the interaction between the chromatic dispersion and nonlinear optical effects.
For example, a citation 1, Y. Miyamoto et al. “Duobinary carrier-suppressed return-to-zero format and its application to 100 GHz-spaced 8×43-Gbit/sec DWDM unrepeatered transmission over 163 km”, Tech. Digest of OFC 2001, paper Tu U4, 2001, discloses a technology relating to a duobinary carrier-suppressed return-to-zero (DCS-RZ) format that modulates a dual mode beat signal with an optical duobinary code.
FIG. 41 is a diagram for explaining the conventional structure of an optical transmitter that uses a DCS-RZ format.
In FIG. 41, a direct current bias is applied to the first push-pull type Mach-Zehnder (MZ) optical intensity modulator 91 so as to realize transmission-null when unmodulated, and the first push-pull type Mach-Zehnder optical intensity modulator 91 is complementarily driven by an electrical sine wave signal having one-half the frequency of the line rate generated by a half frequency divider 92.
The intensity and phase of the CW light output from the single longitudinal mode LD 90, which is the light source of the carrier frequency f0, are simultaneously modulated by the MZ optical intensity modulator 91 using the frequency multiplier function and the phase modulation function of an MZ optical intensity modulator, and a dual mode beat signal having a repetition frequency of B is generated. Here, B is the line rate.
At the second MZ optical intensity modulator 93, the dual mode beat signal is modulated with data using an optical duobinary format. The input NRZ (non-return-to-zero) signal is converted to a pre-coded NRZ code by the precoder circuit 97 that is formed by the logic inversion circuit 94, the exclusive OR circuit 95, and the 1 bit delay circuit 96, and the pre-coded NRZ code is differentially output.
The differential pre-coded NRZ code is amplified by the baseband amplifier 98, and then converted to a complementary ternary electrical duobinary code by the low pass filter (LPF 99) having 3 dB bandwidth of B/4. A direct current bias is applied to the second MZ optical modulator 93 so as to realize transmission-null when unmodulated, and the second MZ optical modulator 93 modulates with a complementary ternary electrical duobinary code to generate a DCS-RZ optically modulated code.
FIGS. 42A through 42F and FIGS. 43A and 43B show an example of the operation of the conventional technology. FIG. 42A shows the binary NRZ signal input generated by the binary NRZ signal generating unit 103. FIG. 42B shows the NRZ data signal output from the logic inversion circuit 94 in the case that a binary NRZ signal is input. FIG. 42C shows the positive-phase signal output from the pre-coding circuit 97 in the case that the output NRZ data signal is input, and the logic thereof is inverted each time a space bit is input as the input NRZ signal. FIG. 42D shows the waveform output from the LPF 99 in the case that the pre-coded signal is input.
As shown by reference numeral 100 in FIG. 41, the logical operation of the LPF 99 is identical to that of a circuit block comprising the 1 bit delay circuit 101 and the analog AND circuit 102. Due to the band limiting function of the LPF 99, the complementary ternary electrical duobinary signal shown by the bold solid line is generated.
FIG. 42E shows the electrical field waveform of the dual mode beat optical signal modulated by the first MZ optical intensity modulator 91 when the CW optical signal from the LD 90, which is the light source, is input. The electrical field waveform forms an optical pulse train where the repetition frequency is equal to the line rate, and whose optical phase is alternate π phase flip for each bit. This dual mode beat optical signal is modulated with the ternary electrical duobinary signal shown in FIG. 42D, and thereby the DCS-RZ code shown in FIG. 42F is generated. The phase is inverted for each mark bit, and thus an RZ intensity modulated optical data signal is obtained.
FIG. 43A shows the optical spectrum of the dual mode beat signal output from the first MZ optical intensity modulator 91. The optical carrier signal component f0 is suppressed, and at the optical frequency fb±(B/2) (where B is the line rate), two longitudinal modes having a frequency spacing of B are generated. The two longitudinal modes are modulated with each of the optical duobinary codes by the second MZ optical modulator 93.
As a result, as shown in FIG. 43B, the optical modulated spectrum of the generated DCS-RZ optical signal is comprised of two optical duobinary signal modulated spectrum arranged at optical frequencies f0±B, the carrier component is completely suppressed, and the optical modulation band is narrowed to 2B. Thereby, the tolerance with respect to chromatic dispersion is double that of the conventional RZ.
The above format suppresses the impairment of the optical duobinary code due to optical nonlinear effects, and thus RZ encoding can be realized while suppressing the broadening of the optical modulation band. Thus, this is suitable as a modulation format in a dense wavelength division multiplexing transmission system.
When considering a wavelength division multiplexing system on a binary RZ intensity modulation code, the optical nonlinear phase shift due to the cross-phase modulation from other channels is strongly depending on the signal pattern, and the interplay between chromatic dispersion and cross-phase modulation (XPM) causes the system performance to deteriorate. In order to mitigate the XPM-induced impairment, T. Miyano et al. propose an RZ-intensity-modulated phase-encoded signal in citation 2, T. Miyano, M. Fukutoku, and K. Hattori, “Suppression of degradation induced by SPM/XPG+GVM transmission using a bit-synchronous intensity modulated DPSK signal”, Digest of OECC2000, Makuhari, paper 14D3-3, pp. 580–581, 2000.
As described above, in a conventional optical transmitter and optical transmission system using an RZ optical intensity modulated format used with phase modulation, generally, optical modulators are necessary for each intensity modulation, phase encoding, and pulse modulation, and these optical modulators are connected in a multi-stage cascade. Thereby, the insertion loss in the modulating unit increases, and the optical output power of the modulating unit decreases. Thus, there are the problems that the optical signal shot noise increases and the SN ratio of the output of the optical transmitting unit degrades.
In addition, in the case of high speed transmission, the relative phase between an electrical data signal and a clock signal for each of the modulators connected in multi-stage must be precisely controlled, and in order to compensate the drift of the phase due to temperature characteristics and the like, a stable phase control must be carried out. Thereby, the problem of the control circuits and the like becoming complicated is made tangible.
Furthermore, in the conventional wavelength division multiplexing system, since two or more optical modulators must be installed for each channel, the number of parts increases, in particular in the case that the number of channel increases in the WDM system. This is a drawback because the cost of the optical transmitter and the optical transmission system using them increases.
At the same time, in the conventional RZ optical transmitter and optical transmission system using a DCS-RZ format, the optical duobinary encoding unit, which carries out the intensity data modulation and phase modulation in data encoding process requires a baseband analog processing circuit (LPF 99 shown in FIG. 41) that generates a ternary opto-electrical signal converted signal depending on the line rate.
As the line rate increases, however, it is difficult to realize the high-speed baseband analog processing in the unit. In order to suppress waveform distortion of the ternary electrical duobinary signal, the waveform distortion due to reflected waves in the rejection band of the LPF 99 must be suppressed. At the same time, in the high frequency band, realizing ideal electrical characteristics is difficult, and in particular, terminating the reflected wave in the rejection band of the filter is difficult. In addition, when realizing the ideal roll-off characteristics of the electrical filter, frequency dependent loss and frequency dispersion of the electrical transmission line and the filters occurs as the line rate increases, and thereby the waveform distortion occurs. Thus, there is the problem that compensation of the waveform becomes difficult.
In addition, the conventional PSK signal that has been modulated using an RZ format can suppress the cross-phase modulation in a wavelength division multiplexing system. However, when considering an increasingly high density of the wavelength division multiplexing system equal to or above 0.4 bits/s/Hz, the optical modulation band spreads four times the line rate, and thus the cross-talk penalty increases. In addition, when considering the high-speed transmission using the conventional technology, it is necessary to increase the operating speed of the baseband signal input into the modulator.
However, as the line rate increases, generally there is a tendency for the breakdown voltage of the electronic device to decrease, and thus this makes difficult to realize high output operation in a driver for driving a modulator or the like becomes difficult. Furthermore, realization of high-speed operation in the pre-coding circuit as well becomes difficult, and it is necessary to redesign and remanufacture the circuit each time the line rate is increased.
It is an object of the present invention to provide an optical transmitter and an optical transmission system wherein decreasing the loss and increasing the speed of the optical modulator is facilitated by using the RZ optical intensity modulation format along with phase modulation. In addition, the invention measures the increasing speed of the analog signal processing by performing a function in the optical carrier frequency domain, which has been carried out by a conventional baseband analog processing circuit. Furthermore, the invention facilitates realization of an amplifier circuit such as a driver circuit by encoding all the electric signals with a simple binary NRZ format.
Furthermore, it is an object of the present invention to provide an optical transmitter and an optical transmission system that use the RZ optical intensity modulation format along with phase modulation, and that make possible simultaneous PSK-ASK conversion of the wavelength division multiplexed signals by using a periodic optical conversion filter, and make possible the elimination of the synchronization function in the active high-speed signal processing by using a passive optical filter.